The present embodiments relate to a cooling system for a magnetic resonance tomography system.
Magnetic resonance tomography systems are imaging apparatuses that, in order to map an examination object, align the nuclear spins of the examination object with a strong outer magnetic field and by a magnetic alternating field, excite the nuclear spins for precession about this alignment. The precession or return of the spins from this excited state into a state with less energy generates, as a response, a magnetic alternating field (e.g., magnetic resonance signal) that is received via antennae.
With the aid of magnetic gradient fields, a spatial encoding is impressed onto the signals, which then permits an assignment of the received signal to a volume element. The received signal is then evaluated, and a three-dimensional imaging representation of the examination object is provided. The generated representation specifies a spatial density distribution of the spins.
Superconducting magnets, which are to be cooled below a transition temperature that lies at a few Kelvin degrees, are typically used to generate the strong outer magnetic field with field strengths of greater than 0.5 or 1 Tesla. Despite state-of-the-art insulating components, a permanent cooling of or heat discharge from the superconducting magnets is provided in order to retain the requisite low temperature. In such cases, pulse tube coolers that reach a temperature gradient from room temperature to the temperature of the liquid helium in one stage, are used inter alia as cooling elements. Due to the large temperature difference, the thermodynamic degree of efficiency is, however, so minimal that significant quantities of heat are to be discharged from the hot side of the pulse tube cooler. Compressors for operating the pulse tube cooler are also considered as the hot side of the pulse tube cooler.
In such cases, typical cooling systems have two cooling circuits. The components to be cooled are thermally coupled to a first cooling circuit. The heat is then discharged from the first cooling circuit to a second cooling circuit that outputs this to the environment. Due to excessively high ambient temperatures, the first cooling circuit and the second cooling circuit are frequently coupled via a heat pump, so that the temperature of the coolant in the first cooling circuit is clearly below the ambient temperature, and the degree of efficiency of the pulse tube cooler is increased and use is first enabled.
With a magnetic resonance tomography system, the cooling system may also be used to cool electrical components.
If a cooling system of this type fails for long periods of time, the superconducting magnet heats up. The further cooling-down of the magnet and the subsequent ramp-up require significant time and a correspondingly expensive operating failure even if prior to exceeding the transition temperature, the magnet may be discharged by a ramp-down so that no consequential damage occurs.
Conversely, however, the cooling system requires significant electrical power, so that the emergency power supply is to be sufficiently large and thus requires correspondingly expensive emergency current systems in order to maintain the cooling with the conventional cooling system. Due to large quantities of helium in the magnets and the large thermal capacity associated therewith, a corresponding emergency cooling system is, therefore, often forgone.